1. Field of the Invention
The present invention relates mainly to a light emitting diode (referred to as "LED" hereinafter) device and, more particularly, to one capable of illuminating an area having a rectangular shape in a high utilization efficiency of a light flux, which is used for such as a flat display LED device or a backlighting LED device in use for a liquid crystal display (referred to as "LCD" hereinafter) device.
2. Brief Description of the Prior Art
FIG. 5 is a perspective view showing a main constitution of a conventional LED lamp, wherein an LED chip 1 is die-bonded on a bottom surface of a horn 2 while a lens 3 is provided above them.
Afore-mentioned LED lamp is equipped with the horn 2 having a circular cone surface and with the lens 3 having a radius of curvature R on a lens surface. As can be seen from FIG. 5, a junction, which constitutes a light emitting center of the LED chip 1, is located on an origin "O" of a three-dimensional rectangular Cartesian coordinate. A main axis of an axial symmetry for the above-mentioned lens surface having the radius of curvature R is located on an X-axis of the rectangular coordinate system formed of X, Y and Z axes. By above-mentioned configuration, a flux of direct incident lights, which are emitted from the LED chip 1 to be incident into a rear surface of the lens 3, and a flux of reflective lights, which are reflected on an inner surface of the horn 2, are spreaded to luminous intensity distribution angles Theta 1 and Theta 2, respectively, and superposed to each other in an X-Y sectional plane of FIG. 6. Above-mentioned luminous intensity distributions are consequently rotated around the X-axis in an direction indicated by an curved arrow as shown in FIG. 6.
Accordingly, the area illuminated by the LED junction, which has a higher luminous intensity than a half value in luminous intensity, exhibits a circular form. Afore-mentioned half value in luminous intensity is defined herein as a luminous intensity that directivity characteristics of the LED exhibit at a half value in angle (referred to as "Theta 1/2"). The directivity characteristics mean herein a three-dimensional luminous intensity distribution of a light flux emitted from the LED junction, which is located on the origin "O" of the coordinate system. Further, afore-mentioned half value in angle is now defined as an inner angle between a direction, wherein the directivity characteristics take a most intensive value, and another direction, wherein the directivity characteristics take a 50% value of the most intensive value.
When a rectangularly shaped region of 1:2 in aspect ratio is illuminated by use of the conventional LED lamp having above-mentioned illumination characteristics, various sorts of configurations have been investigated to even the luminous intensity distribution up to now as shown in FIGS. 7A-7C.
A conventional constitution indicated in FIG. 7A is intended to enlarge the area illuminated with only one piece of LED device by increasing a distance from the LED device to the target area, which circumscribes a circular FIG. 5 of the luminous intensity distribution about a rectangle 4. Herein areas 6, which are hatched with slanting solid lines as shown in FIG. 7A. Represents a loss in flux of the distributed lights.
Another conventional configuration shown in FIG. 7B is intended to widen laterally the figure of the illuminated area by utilizing two pieces of LED lamps corresponding to the shape of the rectangle 4. On the other hand, a still another conventional configuration shown in FIG. 7C is an example, wherein two external optical components 7 such as so called "inner lenses" are auxiliarily equipped above the two LED devices. The illumination area 5 of the conventional example shown in FIG. 7C turns almost rectangular and the loss areas 6 in light flux are further shrinked.
However, when afore-mentioned LED lamps up-to-now are used for illuminating a rectangularly shaped target, they have involved problems that they can merely exhibit a poor utilization efficiency in light flux or, otherwise, their manufacturing costs require much expense.
Namely, the method shown in FIG. 7A has only a low utilization efficiency in light flux and is regarded as an ineffective technology. Although a transforming the external shape of the circular lens is transformed into an elliptic one in order to illuminate the area having an elliptic shape which circumscribes about the rectangularly shaped illumination area improves a little the utilization efficiency, the loss in light flux stays still high.
On the other hand, though the structure shown in FIG. 7B exhibits a better utilization efficiency in light flux compared with the method shown in FIG. 7A, the manufacturing cost turns expensive because number of used LED devices increases. The structure shown in FIG. 7C further increases the utilization efficiency in light flux compared with the structure shown in FIG. 7B, but it raises further the manufacturing costs compared with that of FIG. 7B due to an increase in parts' number of the external optical system, which are auxiliarily equipped.